The present invention generally relates to fabrication of semiconductor devices, and more particularly to a dry cleaning process for cleaning semiconductor substrates for removing contaminants that exist on a surface of the semiconductor substrate.
With advancement in the fine lithographic patterning processes for realizing semiconductor devices having increased integration density, the importance of the process for cleaning the surface of semiconductor substrates increases ever and ever.
In order to meet such a stringent demand for the cleanliness of the surface during the fabrication process of semiconductor devices, dry cleaning processes are proposed as an alternative of conventional wet cleaning processes that use various acids or alkalis together with purified water.
In the various deposition processes and patterning processes for forming a semiconductor integrated circuit on a semiconductor substrate, there is a tendency that the surface of the semiconductor structure is contaminated by elements such as iron (Fe) and sodium (Na) that cause various adversary effects in the operation of the fabricated semiconductor device.
FIGS. 1(A) and 1(B) show a conventional dry cleaning process for removing metal contaminants from a semiconductor surface, wherein FIG. 1(A) shows a surface of a semiconductor substrate 61 contaminated by a contaminant 62 such as Fe and Na, and FIG. 1(B) shows the dry cleaning process that occurs on such a contaminated surface. It should be noted that the illustrated process corresponds to a process described in FIG. 9 of Sugino et al, IEICE, Trans. Electron., vol.E75-C, no.7, July, 1992, which is incorporated herein as reference.
In the illustrated conventional process, a halogen gas such as chlorine, hydrogen chloride, fluorine, hydrogen fluoride, and the like, are supplied to the contaminated surface as a dry cleaning gas, while simultaneously maintaining the semiconductor substrate 61 at an elevated temperature for activating the halogen molecules. One may apply ultraviolet radiation as necessary for assisting the activation of the halogen molecules. The halogen molecules thus activated in turn react with the semiconductor substrate 61 to cause a slight etching of the semiconductor substrate 61, and the contaminants 62 are removed simultaneously to the dry etching process thus caused for example in the form of halides such as FeCl.sub.3. When applying the foregoing dry cleaning process to the fabrication of semiconductor devices, it is generally necessary to set the cleaning condition such that the amount of etching occurring at the surface of the semiconductor substrate is held to a minimum. This requirement is particularly acute in the fabrication of semiconductor devices having extremely fine patterns.
The foregoing conventional dry cleaning process is effective in the system wherein a substantial etching reaction occurs at the semiconductor surface by the halogen gas as in the case where a chlorine gas is applied to a silicon substrate. In such a case, it is believed that the dry cleaning occurs as a result of the etching of the surface. Thus, there is a demand for a dry cleaning process wherein no substantial etching occurs in the surface that is subjected to the cleaning.
Meanwhile, the foregoing process of applying chlorine gas is inapplicable for cleaning the surface of a stable silicon oxide film that is deposited by sputtering or CVD or formed by a thermal oxidation process, as there occurs no substantial etching reaction between the chlorine gas and the silicon oxide film thus formed.
FIGS. 2(A) and 2(B) are diagrams comparing the effect of conventional dry cleaning process, wherein FIG. 2(A) compares the result of the experiment where Fe is applied intentionally on the exposed silicon surface as contaminant and removed subsequently by the dry etching process, with the result of a similar experiment wherein Fe is applied and removed on and from a silicon oxide surface, instead of the exposed silicon surface. Further, FIG. 2(B) shows the result of similar experiments except that the contaminant element is changed from Fe to Na. In FIGS. 2(A) and 2(B), the hatched bar graph shows the result for the silicon oxide film, while the blank bar graph shows the result for the exposed silicon surface. Further, it will be noted that the arrow A in FIGS. 2(A) and 2(B) indicates the experiment conducted on the silicon oxide film, while the arrow B indicates the experiment conducted on the silicon substrate at a dry cleaning temperature of 200.degree. C. Further, the arrow C in both drawings indicates the experiment conducted on the silicon substrate with a dry cleaning temperature of 400.degree. C.
From FIGS. 2(A) and 2(B), it will be noted that the surface of the silicon substrate is cleaned significantly with the dry cleaning process as indicated in the blank bars, irrespective of whether the contaminant is Fe or Na, while it will be further noted that the surface of the silicon oxide film remains substantially uncleaned as indicated by the hatched bars. It is believed that the foregoing result indicates the strong and stable chemical bond established between the oxygen forming the silicon oxide film and the metal elements such as Fe or Na, and such a strong chemical bond suppresses the etching reaction of the surface.
In order to reduce the bonding energy between oxygen and metal elements and facilitate the etching reaction, the inventors of the present invention proposed previously a dry cleaning process as disclosed in the Japanese Patent Application 2-065105, wherein a silicon chloride (SiCl.sub.4) gas is added to the chloride etching gas.
FIG. 3 shows the foregoing dry cleaning process, wherein FIG. 3 shows the process as applied to the structure that includes a silicon substrate 71 covered by a silicon oxide film 72, wherein it will be noted that a contact hole 73 is formed in the silicon oxide film 72.
In the process shown in FIG. 3, SiCl.sub.4 is admixed into the chlorine cleaning gas as mentioned before, to facilitate a reaction between the metal contaminant and chlorine radicals. Thereby, metal chlorides such as FeCl.sub.3 or FeCl.sub.x are produced in the gaseous form as a result of the reaction between Cl contained in SiCl.sub.4 and Fe, wherein the bonding energy between Fe and oxygen in the silicon oxide film is reduced. Thereby, the metal chloride thus formed is removed from the surface of the silicon oxide film 72. There, SiCl.sub.4 acts as a catalyst. In order to enhance the foregoing catalytic effect, ultraviolet radiation may be applied to the SiCl.sub.4 gas. Thereby, the metal contaminants are effectively removed from the surface of the silicon oxide film.
In the foregoing approach, on the other hand, although the process is effective in reducing the contaminants from the silicon oxide surface, it was observed that a part of the SiCl.sub.4 gas, admixed into the chlorine cleaning gas, may reach the contact hole 73 and cause an unwanted deposition of an amorphous silicon film 74 on the exposed surface of the substrate 71.